Attempts have been made in the past to use adhesive compositions to bond together foundry elements of the type described into a composite molding assembly. As used in this specification, the term "foundry element" means molding elements made of aggregate foundry mixes, such as cores and molds. Molding elements made of other materials such as ceramic may also be used.
Such prior art uses of binder type compositions as foundry adhesives have encountered various problems and have resulted in a number of deficiencies.
One problem is in the difficulty of controlling gel times, such that the adhesives cure too rapidly or too slowly for practicality.
A second problem concerns difficulty of application due to the resin component being too viscous, preventing easy mixing and application, and the hardener component being too thin, so that it penetrates too deeply into the mold or core elements, leaving insufficient paste at the interface for bonding. U.S. Pat. No. 3,993,284 discloses a foundry core paste having a viscosity of 30,000 centipoise, making it difficult to mix and apply. Substantial differences in viscosities between the components also result in poor mixing characteristics leading to unpredictable gel times.
Self-setting adhesive foundry pastes are also known, such as the urethane paste of U.S. Pat. No. 4,692,479. However, this paste is deficient in that it is not water-cleanable, contains nitrogen and isocyanate and may generate byproducts which may cause casting defects.
Other deficiencies of the prior art include unworkable consistencies; foaming and other characteristics causing dimensional changes after application; low tensile strengths; resoftening with heat; deterioration of adhesive upon water absorption, and the like.
Other prior art practices include the use of relatively expensive hot melt adhesives which are prone to thermal instability (resoftening or other loss of tensile strength) when subjected to heat from the molten metals being cast into shapes or other processing operations. Hot melt adhesives also may resoften upon core wash and oven drying of the molding elements. Such thermal instability allows the glued parts to shift, thereby ruining the tolerances of the cast metal shape. Softening of the adhesive also may result in run out of the molten metal, which similarly may destroy the tolerances of the cast shapes. Run out results from an inadequate adhesive seal between the molding assembly elements and also may result from an improper consistency of the applied adhesive. Another problem with hot melt adhesives is they are expensive and hazardous to handle and the equipment used for their application is subject to considerable down time and maintenance.
Prior art practices also include the use of air or oven dried adhesives, such as filled dextrin or lignosulfonate pastes and sodium silicate/silica flour/clay pastes. This class of adhesives is slow to cure and therefore significantly limits production rates. A delay of sometimes as much as 10 to 15 hours after gluing together the elements of the molding assembly may be necessary before molten metal can be poured into such an assembly. It has long been recognized that the elimination of such time delays would significantly increase production rates.
The use of lactone-curable and ester-curable alkaline phenolic resin resoles that cure at room temperature has been described. These resins are useful as binders for sand for making molds for foundry applications. See U.S. Pat. Nos. 4,426,467; 4,468,359, and 4,474,904, all of which are herein incorporated by reference.
By "phenolic resin" is meant the reaction products of a phenol with an aldehyde in which the final mixture of molecules in the reaction products is dependent upon the specific reactants selected, the starting ratio of these reactants, and the conditions of the reaction (for example, the type of catalyst, the time and temperature of the reaction, the solvents and/or other ingredients present, and so forth). The reaction product, that is, the phenolic resin, will be a mixture of different molecules and may contain in widely varying ratios addition products, condensation products, and unreacted reactants such as unreacted phenol and/or unreacted aldehyde. By "addition product" is meant reaction products in which an organic group has been substituted for at least one hydrogen of a previously unreacted phenol or of a condensation product. By "condensation product" is meant reaction products with two or more benzene rings.